It is well known that coatings or paints incorporating silicone resins have high heat resistance. Heretofore, silicone-containing coatings were generally liquid coatings employing silicone as a binder. Most prior art silicone-containing coatings are solvent-based and have the disadvantage of high VOCs. Water-borne silicone-containing coatings are also known, but even these generally contain some solvent. It is an object of the present invention to provide a silicone resin-containing coating powders. Powder coatings have the advantage of having no organic solvents. Furthermore, overspray coating powder is completely recoverable and reusable.
Liquid silicone resin-containing coatings are known to contain silicone resin-compatible co-binders, such as alkyds, acrylics, epoxies, melamine resins, polyesters and polyurethanes. The most common co-binders in liquid silicone resin-containing coatings are polyesters. The co-binder resins are used to provide initial properties of the coating. Silicone resins cure slowly and at elevated temperatures. Co-binders, such as polyesters, are more rapidly curable and are curable at lower temperatures.
Silicone-based coating powders have been previously produced, e.g., U.S. Pat. No. 3,170,890, the teachings of which are incorporated herein by reference, using epoxy resins as co-resins; however, the finishes produced from these resins have poor weatherability.
U.S. Pat. No. 4,879,344 to Woo et al., the teachings of which are incorporated herein by reference, describes coating powders which are a mixture of a hydroxy-functional acrylic resin and silicone. It is found, however, that in using hydroxy-functional acrylic resins, out-gassing occurs when the coating is fused and/or cured onto the substrate.
U.S. Pat. No. 4,968,751 to Miles et al. is directed to acrylic/silicone resins in which the acrylic resin has significant hydroxyl functionality so as to react with the silicone resin, plus phosphonium salt to catalyze the reaction between the hydroxyl functional acrylic resin and the silicone resin. This reaction out-gasses water.
U.S. Pat. No. 5,280,098 is directed to an epoxy-functional silicone resin. Compositions may also contain acrylic resin. The silicone resin is very highly substituted with epoxy-containing moieties as well as other organic moieties. The silicone resin has no silanol (Si--O--H) functionality.